Pressure mounted on Monday against Catalan vows to break away from Spain after hundreds of thousands of protesters rallied to defend national unity, but the region's leader maintained his threat to formally declare independence possibly within days.

The protests followed days of soaring tensions after police cracked down on voters during a banned October 1 Catalan independence referendum, prompting separatist leaders to warn they would unilaterally break away from Spain.

Catalan separatists have called on regional president Carles Puigdemont to declare independence in defiance of the central government when he addresses the regional parliament on Tuesday evening.

He hinted in an interview on Sunday that the region would go ahead and declare independence if Madrid continues to refuse dialogue.

"We have left the door open to mediation and we have said yes to so many mediation options that have been proposed," he told Catalan television channel TV3.

"The days are going by and if the Spanish state does not give a positive response, we will do what we set out to do."

'Silent majority'
Hundreds of thousands of flag-waving demonstrators, calling themselves a "silent majority", packed central Barcelona on Sunday to protest against the plan, which has sparked the country's worst political crisis in a generation.

Around 350,000 people attended the rally, municipal police said, while organisers put turnout at between 930,000 and 950,000.

Some protesters called for Puigdemont to go to jail for holding the independence vote.

Others called for dialogue. The slogan for the demonstration -- organised by the Societat Civil Catalana, the main anti-independence group in Catalonia -- was: "Enough, let's recover good sense!"

Tentative signs emerged last week that the two sides may be seeking to defuse the crisis after Madrid offered an apology to Catalans injured by police during the vote.

But uncertainty still haunts the country, with the standoff also raising deep concerns in the European Union as it continues to grapple with Britain's planned exit from the bloc.

But Rajoy assured Catalan leaders that there "is still time" to backtrack and avoid the imposition of direct rule from Madrid.

Kofi Annan, in his role as chairman of The Elders, a group of notable public figures formed in 2007 to promote peace, said: "The constitutional crisis that is unfolding in Spain calls for consultation and not confrontation.

"I urge the Spanish government and the regional government of Catalonia to renew their commitment to a resolution through dialogue," the former UN chief added.

Recent opinion polls indicate that Catalans are split on independence, although regional leaders said police violence during the referendum turned many against Madrid.

Police said 700,000 people joined a pro-independence protest in Barcelona two days after the vote.

'Traitors'
With its own language and cultural traditions, demands for independence in Catalonia date back centuries but have surged during recent years of economic hardship.

Catalonia, a northeastern region about the size of Belgium, is home to 7.5 million people and accounts for a fifth of Spain's economy.

At Sunday's rally, demonstrators cheered and applauded when a national police helicopter flew over and some people shook the hands of national police officers to thank them for their efforts to stop the referendum.

Protesters jeered members of Catalonia's regional police force, the Mossos d'Esquadra, which had largely ignored a court order to close polling stations and seize ballot boxes during the referendum.

Dozens of protesters surrounded two Mossos vans and called the officers standing on guard in front of them "traitors".

The Catalan government on Friday published final results from the referendum indicating that 90 percent of voters backed the proposal to break away from Spain.

Turnout was 43 percent as Catalans who reject independence largely boycotted the poll.

The vote was not held according to official electoral standards as there were no regular voter lists, electoral commission or observers.